1. Field of the Invention
The present invention relates generally to the field of emission control equipment for boilers, heaters, kilns, or other flue gas-, or combustion gas-, generating devices (e.g., those located at power plants, processing plants) and, in particular to a new and useful method and apparatus designed to improve the water supplied to non-calcium-based, aqueous wet SOx scrubbers. In another embodiment, the present invention relates to a system and method for softening water for use in non-calcium-based, aqueous wet SOx scrubbers.
2. Description of the Related Art
Sulfur appears in the life cycle of most plants and animals. Most sulfur emitted to the atmosphere originates in the form of hydrogen sulfide from the decay of organic matter. These emissions slowly oxidize to sulfur dioxide (SO2). Under atmospheric conditions, SO2 is a reactive, acrid gas that can be rapidly assimilated back to the environment. However, the combustion of fossil fuels, in which large quantities of SO2 are emitted to relatively small portions of the atmosphere, can stress the ecosystem in the path of these emissions. As used herein, SO2 and SO3 emissions may generally be referred to as sulfur oxides or SOx emissions.
Man is responsible for the majority of the SO2 emitted to the atmosphere. Annual worldwide emissions are generally accepted to be over 160 million tons, nearly half of which are from industrial sources. The two principal industrial sources are fossil fuel combustion and metallurgical ore refining.
When gaseous SO2 combines with liquid (l) water, it forms a dilute aqueous solution of sulfurous acid (H2SO3). Sulfurous acid can easily oxidize in the atmosphere to form sulfuric acid (H2SO4). Dilute sulfuric acid is a major constituent of acid rain. Nitric acid is the other major acidic constituent of acid rain. The respective reactions are written as follows:SO(g)+H2O(l)→H2SO3(aq)  (1)O2(g)+2H2SO3(aq)→2H2SO4(aq)  (2)
SO2 can also oxidize in the atmosphere to produce gaseous sulfur trioxide (SO3). Sulfur trioxide reactions are written as follows:2SO2(g)+O2(g)→2SO3(g)  (3)SO3(g)+H2O(g)→H2SO4(l)  (4)
While Equations 1 and 2 describe the mechanism by which SO2 is converted to sulfuric acid in acid rain, Equations 3 and 4 characterize dry deposition of acidified dust particles and aerosols.
The pH scale, a measure of the degree of acidity or alkalinity, is the method used to quantify the acidity of acid rain.
Pure water has a pH of 7 and is defined as neutral, while lower values are defined as acidic and higher values as alkaline. If rainwater contained no sulfuric or nitric acid, its pH would be approximately 5.7 due to absorption of carbon dioxide (CO2) from the atmosphere. The contributions of man-made SO2 and nitrogen oxides (NOx) further reduce the pH of rainwater. No uniformly accepted definition exists as to what pH constitutes acid rain. Some authorities believe that a pH of about 4.6 is sufficient to cause sustained damage to lakes and forests in the northeastern portion of North America and in the Black Forest region of Europe.
SO2 Emissions Regulations:
Legislative action has been responsible for most industrial SO2 controls. Major landmark regulations include the Clean Air Act Amendments of 1970, 1977 and 1990 in the United States (U.S.), the Stationary Emissions Standards of 1970 in Japan, and the 1983 SO2 Emissions Regulations of the Federal Republic of Germany. Since the mid-1980s, SO2 emissions regulations have been implemented in most other industrialized nations and many developing nations.
SO2 Control:
Most utilities have adopted one of two strategies for SO2 control, either switching to low sulfur coal or installing scrubbers. A variety of SO2 control processes and technologies are in use and others are in various stages of development. Commercialized processes include wet, semidry (slurry spray with drying) and completely dry processes. The wet flue gas desulfurization (WFGD) scrubber is the dominant worldwide technology for the control of SO2 from utility power plants, with approximately 85% of the installed capacity, although the dry flue gas desulfurization (DFGD) systems are also used for selected lower sulfur applications.
Total annual SO2 emissions in the U.S., including electric utility SO2 emissions, have declined since 1970 as various regulations have been adopted. During the same period, electricity generation from coal has almost tripled (see Table 1 below).
TABLE 1U.S. SO2 Emissions and Coal-Fired Power GenerationCoal FiredUtilityTotal U.S.Utility SO2 Generation YearSO2 106 t/yr106 t/yr1012 kWh197031170.7198026171.2199023161.6200016112.0
A significant portion of this emissions reduction has been the result of switching to low sulfur coal, predominantly from the western U.S. In 1970 virtually all of the utility coal came from the eastern, higher sulfur coal fields, while by 2000 approximately half of the coal came from western low sulfur sources. Slightly less than two-thirds of SO2 emission reductions have been attributed to fuel switching while over a third has been through the installation of flue gas desulfurization systems, predominantly wet scrubbers. More than 50% of the U.S. coal-fired capacity already has FGD systems installed and operating. This may increase to more than 80% over the next decade and a half as existing regulations are implemented and proposed regulations are adopted.
Wet Scrubbers—Reagents:
Wet scrubbing processes are often categorized by reagent and other process parameters. The primary reagent used in wet scrubbers is limestone. However, any alkaline reagent can be used, especially where site-specific economics provide an advantage. Other common reagents are lime (CaO), magnesium enhanced lime (MgO and CaO), ammonia (NH3), and sodium carbonate (Na2CO3). In the case of sodium carbonate (Na2CO3), scrubbers based on this chemistry suffer scaling problems due to the presence of dissolved calcium in the makeup water. Scaling problems require unit outages for cleaning WFGD absorbers every 6 to 8 months. This puts soda based scrubbers at a disadvantage compared to limestone based WFGD.
The process by which soda, or sodium carbonate, wet scrubbers operate is well known to those of skill in the art. For example, one suitable reaction process is detailed in Sulfur Oxides Control Technology Series: Flue Gas Desulfurization Dual Alkali Process (EPA Document 625/8-80-004, October 1980), the text of which is hereby incorporated by reference as though fully set forth herein in its entirety.
In one instance, natural fresh water is used as the base stock for the raw makeup water. Such waters, prior to treatment, contain varying amounts of inorganic impurities, the most common being dissolved calcium, magnesium, iron, carbonates, and sulfates in ionic form. Water that has not been treated to remove any of these impurities is sometimes referred to as raw water. The total carbonate content in the raw water is referred to informally as the total alkalinity. The hardness of the water is in turn determined directly by the total amount of calcium and magnesium. The term generally refers to the negative effect that these ions have on the ability of soaps and detergents to lather in hard water. In the context of a wet scrubber that uses sodium hydroxide, sodium carbonate, or sodium bicarbonate as a reagent to scrub sulfur dioxide from a flue gas, or combustion gas, the concern about the hardness constituents in the raw water is that as raw water becomes exposed to the scrubber solutions inside the wet scrubber, the calcium ions will react with carbonate ions, sulfite and bisulfite ions and sulfate ions to form solid calcium carbonate, solid calcium sulfite, and solid calcium sulfate. Such solid compounds tend to deposit on the internals of the scrubber causing scaling sufficient to render the scrubber inoperable. Such a situation requires, at some point, the operator of the facility to shut such a “fouled” scrubber down long enough to enter the scrubber and manually clean it out. Such an operation involves significant time in lost production and physical cleaning expenses. To mitigate such detrimental consequences, an operator attempts to reduce the amount of hardness in the raw water by treating that water prior to use in such a scrubber. One such conventional treatment method is depicted in FIG. 1.
As is illustrated in FIG. 1, conventional system 100 includes floc supply line 102, sodium carbonate (Na2CO3) solution supply line 103, raw water supply line 104 and lime supply 106. Lime supply 106 supplies lime to detention slaker 108. Detention slaker 108 includes therein at least one agitating device (e.g., a mixer) and is provided with a water supply line to permit the mixing of the lime from lime supply 106 with water to yield a lime slurry that is supplied, via lime slurry supply line 110, to precipitator/crystallizer 112. Also supplied to precipitator/crystallizer 112 are floc via floc supply line 102, sodium carbonate solution via sodium carbonate supply line 103 and raw water via raw water supply line 104. Precipitator/crystallizer 112 also includes at least one agitating device (e.g., a mixer) to facilitate the mixing of the sodium carbonate solution, floc, raw water and lime slurry. Once any undesirable solids are permitted to “settle out” and/or precipitate to the bottom of precipitator/crystallizer 112, this treated solution of sodium carbonate solution, lime, raw water and floc is supplied via supply line 114 to a settler/thickener 116. In settler/thickener 116 the once-treated mixture of sodium carbonate solution, lime, raw water and floc is further treated to remove additional unwanted solid particles via the use of one or more agitating devices (e.g., a mixer). The solids generated by this process are then supplied, with an appropriate amount of solution, to a sludge pond 122, via supply line 118, to permit further settling and reclamation of the solids contained in such a waste solution. Additionally, or in some cases optionally, a portion of the solids generated by settler/thickener 116 are re-supplied, with an appropriate amount of solution, to precipitator/crystallizer 112 via supply line 120 to supply seed crystals for the precipitation stage.
Once any undesirable solids are permitted to “settle out” and/or precipitate to the bottom of settle/thickener 116, the twice-treated solution of sodium carbonate solution, lime, raw water and floc is supplied via supply line 124 to treated water tank 126. In treated water tank 126 the twice treated solution of sodium carbonate solution, lime, raw water and floc is combined with sulfuric acid (H2SO4) from sulfuric acid tank 130 via sulfuric acid supply line 132. This combination of twice-treated solution and sulfuric acid is then further agitated via a suitable device (e.g., a mixer) until a desired pH is obtained. Once this occurs, the suitably treated solution is supplied to a wet scrubber via supply line 134.
As is known to those of skill in the art, lime softening works by raising the pH of the raw water and causing the bicarbonate to convert to carbonate and then precipitating the calcium as calcium carbonate. Once the pH rises to above about 10, magnesium starts to precipitate as magnesium hydroxide. FIG. 2 is a plot of the calcium and magnesium concentration of a raw water that contains only calcium and magnesium carbonates and bicarbonates. Note that the calcium concentration actually begins to rise as the magnesium drops above a pH of 10.
Given the data contained in FIG. 2, the only way that one could achieve both low magnesium and low calcium values would be to perform the softening in two stages. First, one has to raise the pH to 11 and after separating out the precipitates, lower the pH back to 10 with, for example, sulfuric acid to precipitate out the excess calcium as calcium carbonate.
If raw water contains only calcium and magnesium sulfates, then lime softening will remove no calcium at all but the magnesium will be removed at a pH above 10. That is confirmed via the data shown in FIG. 3. So lime softening does have the capacity to reduce calcium concentration if the raw water contains principally calcium carbonate. But to remove both calcium and magnesium, the system must be operated in two stages. Removing the magnesium at a pH above 11 and then reducing the calcium at a pH around 10.
Given the above, a need exists for a method and/or apparatus that provides for an efficient manner by which to remove the undesirable calcium ions from the raw water used for make-up in non-calcium-based, aqueous wet SOx scrubbers.